Various systems are known for measuring the refractive index of a test object, e.g. an optical fiber or a preform used in its manufacture, in order to determine whether that index conforms to existing requirements. Devices used for this purpose include refractometers of the Pulfrich and Abbe types as well as spectrometers; see, for example, pages 17, 22 and 23 of "Fundamentals of Optics" by Francis A. Jenkins and Harvey E. White, published 1957 by McGraw-Hill Book Company, Inc.
All these conventional devices are limited to samples of certain shape (e.g. prismatic) taken from the body to be tested. Thus, they cannot be utilized for the continuous measurement of the refractive index of a body in production, e.g. an optical preform manufactured by the coating of an inner or outer cylindrical surface of a vitreous support with a similar material in a succession of layers, as by the technique of chemical-vapor deposition (CVD). Nor do these devices facilitate the determination of the thickness of the transparent body or layer of interest.
It has already been proposed to determine the refractive index of a preform during manufacture thereof by measuring its surface temperature. Such a method, of course, requires the surface being coated to be accessible and can therefore not be employed when the vitreous material is being deposited on the inner surface of a tubular support. Again, there is no possibility with this method to determine the thickness of the coating or of any layer thereof.